By Jon Herskovitz

JOHANNESBURG (Reuters) - A South African province home to thousands of elephants is planning a birth control campaign for the pachyderms to prevent a population explosion that could threaten plants and wildlife.

Unlike other parts of Africa where elephant stocks have dwindled to dangerously low levels due to poaching and a loss of habitat, South Africa has seen its populations steadily grow through conservation, with the country pressed for room to house the massive animals with hefty diets.

KwaZulu-Natal province, in the southeast, is looking to expand a project running for more than a decade where elephants populations have been controlled by injecting cows with a vaccine that triggers an immune system response to block sperm reception.

"Slowing the growth rate will allow time to be gained to achieve other biodiversity objectives, such as land expansion, without having to cull the elephants," said Catherine Hanekom, an ecologist for Ezemvelo KZN Wildlife.

South Africa, which had just over 100 elephants nearly a century ago, now has more than 20,000, studies said.

The overpopulation problem is the most dire in neighboring Botswana, home to at least 133,000 elephants, where vast forests have been lost to satiate their appetites. With a human population of 2 million, it has the highest elephant-to-people ratio in Africa, at one for every 14 people.

Adult elephants consume about 100 to 300 kgs (220 to 660 pounds) of food a day and most elephants in South Africa are in fenced-in reserves where vegetation could be decimated if populations grow too large.

"Because we have taken away opportunities, they don't have the chance to remedy the overpopulation naturally as they would through migration," said Audrey Delsink Kettles, an elephant ecologist who has been leading studies for years on contraception at Makalali Private Game Reserve.

Testing of the vaccine, administered by dart and requiring an annual booster, has been conducted at 14 small reserves. Studies have shown it is reversible, nearly 100 percent effective and has no adverse impact on elephant health or behavior, Kettles said.

Contraception is seen as a humane alternative for controlling populations over the other main options of culling herds or moving them vast distances to areas with more food.

The Humane Society of the United States and Humane Society International have backed the vaccine.

"Failure to control the reproduction of the species ... leads to a population that exceeds the carrying capacity of the reserve and to habitat degradation," they said in statement.

(Reporting by Jon Herskovitz, editing by Paul Casciato)